Conventionally, image pickup apparatuses such as compact digital cameras perform contrast-based focus detection using an image pickup device. Contrast-based focus detection enables accurate automatic focus control using image signals output from an image pickup device. For this reason, it has been proposed that in digital single-lens reflex cameras, after mirror lock-up after phase difference-based focus detection, contrast-based focus detection is performed in the same focus detection area.
As described above, contrast-based focus detection enables more accurate automatic focus control than in phase difference-based focus detection. A reason that can be given for this is that in contrast-based focus detection, outputs from an image pickup device are used, whereas in phase difference-based focus detection, focus adjustment units are on different optical paths, and aberration resulting from mechanical optical-path difference and different pupils is corrected for.
FIG. 6 is a diagram useful in explaining an operation in a case where focus control is performed in a selected focus detection area. In the figure, the abscissa indicates the camera-to-subject distance, and after mirror lock-up after a selected subject is focused on by an AF unit in a mirror-down state, focus scan is performed before and after a focal position so as to detect a contrast peak. In the figure, the arrow A indicates a movement of a lens, a face at a position indicated by the arrow B is a target focal position, and the waveform C indicates a peak of contrast evaluation values (PTL (Patent Literature) 1).
Also, there has been proposed the technique that part of pixels of an image pickup device are provided with different optical characteristics from those of other pixels and used for focus detection so as to dispense with a secondary optical system for conventional focus detection (PTL (Patent Literature) 2).